For quite a long time now the convergence of the product sectors of consumer electronics (HiFi, video, audio) on one side and personal computers on the other have been trumpeted under the catchword multimedia and have actually been propelled by many manufacturers from both camps. The merging of the two product sectors means that work concerned with the subject of data exchange between the equipment of the different product sectors or else between the equipment within one product sector is becoming more and more significant. This is also apparent from the efforts for standardization with regard to this subject, which are already well advanced. Specifically, the so-called IEEE1394 serial bus already provides an internationally standardized and very widely accepted bus for data exchange between terminals from both product groups. The precise designation of the afore-mentioned standard is:
IEEE1394 standard for high performance serial bus, (IEEE) STD 1394/1995, IEEE New York, Aug. 1996.
The invention that is to be described here is concerned with the bus management. This IEEE1394 bus provides the service of isochronous data transfer as well as asynchronous data transfer. In this connection isochronous data transfer means that data to be transmitted arrives regularly at a data source the data also arriving with approximately the same size each time. Examples of such data sources are video recorders or camcorders, DVD players, audio devices such as CD players or MP3 players and the like. The asynchronous transfer mode is used for all the other data transfers including control commands and configuration messages, which are not as critical as in the case of streaming video or audio.
The specification of the IEEE1394 serial bus comprises a series of criteria, which are highly significant for products from the sector of consumer electronics:                Virtually freely selectable bus topology (e.g. chain, tree . . . ) with up to 63 terminals,        Bit serial data transmission over a cable with 4 or 6 conductors with the maximum distance of 4.5 meters between two pieces of equipment,        Transmission rates of up to 400 MBit/s at present,        Terminals can be connected and disconnected during operation (life insertion).        
In order to produce an IEEE1394 interface it is necessary to implement two layers of the IEEE1394 standard using hardware: These are the physical layer and data link layer which are known from the OSI/IEC-7-Layer reference model of data communication. The connection to the bus is managed with the physical layer, while essential parts of the bus protocol are implemented in the data link layer. Since the galvanic separation between the physical layer and the data link layer is provided in the 1394 standard the implementation is generally effected using separate IC's.
In a given configuration of the 1394 bus the bus management assures that the bus is shared for transportation of isochronous data packets as well as asynchronous data packets in a fair manner, this is basically done by introducing a so-called cycle master in the network. It is a bus node with cycle master capability that will be assigned as cycle master. This bus node provides for time synchronisation for all bus nodes in the network by means of transmitting in a regular intervals cycle start packets. Every bus node has to resynchronise its time clock after reception of a cycle start packet. To assure this synchronization process, in each of the bus nodes there is a cycle timer running, that generates with its own time clock local cycle synchronization events. After the local cycle synchronization event a bus node will wait for the reception of the cycle start packet before making own transmission requests. After reception of a cycle start packet a bus node will wait for a short isochronous gap before generating a transmission request for isochronous data if any has become due. Each of the nodes having made isochronous transmission requests will get access to the bus during the nominal cycle period. This is assured by an isochronous resource manager that is also established in the 1394 standard. If the data transport capacity of the 1394 bus has not been seized by the isochronous manager, all the other nodes having made asynchronous transmission requests can access to the bus after elapse of a certain amount of time called sub-action gap. The rest of the cycle period is, therefore, dedicated for asynchronous transmissions.
The shortly explained bus management procedure where the isochronous data transport capacity is guaranteed and the remaining data transport capacity is reserved for asynchronous transmissions has the following drawback:
In case of the use of the IEEE1394 bus for serial communication between devices that do not need to transfer isochronous data, the bus management with the cycle master and dedicated cycle periods is not the most efficient one. Each bus node has to wait for the reception of the cycle start packet after each local cycle synchronization event and also it has to wait for the sub-action gab before it can request asynchronous transmission. This is somehow a waste of time namely the transmission time for the cycle start packet as well as the waiting time for the sub-action gap. During this time in each of the cycle periods no data transport can take place.